Many plasma-based chemical analysis instruments utilize a radio frequency (RF, 10-100 MHz) inductively coupled plasma (ICP) and utilize argon as the process gas. While argon ICP is a relatively mature and effective technology, supplying argon for these instruments adds a significant cost of ownership, and is subject to the availability of purified argon gas. To address the needs of new and emerging markets (rural, portable, developing world, etc.) recent interest is being directed to producing instruments of similar function that can operate with a less expensive, more available gas such as molecular nitrogen (N2).
Due to the differences in the properties of argon and nitrogen plasmas, microwave radiation in the range of several GHz (as opposed to RF radiation) is often better suited for energizing nitrogen plasmas. An instrument configured for generating such plasmas may include a power subsystem that delivers microwave power to a process gas flowing through a tube (or set of nested tubes). The microwave radiation ionizes the gas into plasma. The resulting microwave-induced plasma (MIP) may be utilized to heat, evaporate/dry/desolvate, atomize, electronically excite and ionize analyte materials, thereby enabling subsequent detection and analysis of emitted light or ions.
The microwave power subsystem may include a microwave power source and an electromagnetically resonant structure coupled by a power transfer element such as a free-space waveguide or a coaxial cable. This system may also require additional elements for impedance matching of its various components, including manual and/or servo-locked stub tuners and movable shorting walls. Also, microwave isolators and circulators are often included to protect the power source from reflected power in the case of an impedance mismatch situation.
Known microwave power subsystems are associated with disadvantages. Due to their multi-component configuration, they are relatively bulky and costly. Also, automated tuning algorithms may be needed to achieve and maintain acceptable performance, and the tuning process, in turn, involves moving parts. These factors detract from suitability in the sought-after “in the field” applications market. In addition, due to the high volume and/or area of the multi-component design there is significant power dissipation (energy loss), mostly by the walls of the guiding structure. In addition, there is a mismatch between the electromagnetic (EM) field patterns in commonly used resonant structures (e.g., rectangular waveguides or coaxial cables) and the field patterns that tend to produce the cylindrical or toroidal plasma symmetry that is preferred for spectroscopy/spectrometry applications. These mismatches contribute to compromised performance and stability in current designs, which remains a topic of ongoing research.
Therefore, there continues to be a need for improved systems, devices and methods for generating MIP for various applications.